The present invention relates to a pressure sensor element comprising built-in temperature measuring means, in particular a ceramic capacitive pressure sensor element having capacitor electrodes on tile bottom side of a house part and on the top side of a diaphragm.
A ceramic capacitive sensor element 1 for sensing pressures is usually built of mainly two parts, see FIG. 9. These parts comprise a stable circular base plate 3 having a diameter of typically 20-30 mm and a thickness of typically 4-5 mm, also called a housing or house part, and a thinner circular plate 5, also called a diaphragm, applied to one of the large surfaces of the base plate 3 and joined thereto by means of for example glass joints 6 at its circular edge. The diaphragm 5 is attached so that its central portion can move, bend or be deflected in relation to the base plate, i.e., the basically flat shape of the diaphragm can change for varying pressures acting thereon. The diaphragm has the same diameter as the base plate and has a thickness, which is adapted to the magnitude of the load, i.e., the pressure, to which the diaphragm is intended to be subjected. The change of the position of the central portion of the diaphragm 5 is detected as a change of the capacitance between two parallel and opposite electrodes 7, 9 of, e.g., gold, which are applied by means of thin film methods on the central portions of the inner, opposite surfaces of the base plate 3 and the diaphragm 5 respectively. In the measurement of pressure the variable searched for is the pressure Pmeas, which acts on the bottom, free surface of the diaphragm 5, and it is measured in relation to a reference pressure Pref acting on the inner surface of the diaphragm, i.e., the surface facing the base plate 3. Temperature measurement elements can be applied to the interior side of the diaphragm 5, see German publication document DE-A1 41 36 999. However, this involves a large disadvantage due to the fact that additional surface coatings on the diaphragm will always to some extent influence the mechanical characteristics of the diaphragm and in particular temperature induced movements in the diaphragm can increase. This can be particularly embarrassing when measuring using thin diaphragms.
It is an object of the invention to provide a pressure sensor comprising integrated measurement of temperature and having a high accuracy and repeatability.
It is another object of the invention to provide a capacitive pressure sensor, which comprises temperature measurement elements which have a minimal influence of the mechanical characteristics of the measurement diaphragm and also on the electric fields at the capacitor electrodes of the pressure sensor.
The general problem solved by the invention is thus how to arrange temperature measurement means inside a compact ceramic pressure sensor of the capacitive type allowing an accurate temperature measurement at the place where it is needed, i.e. as near the capacitor electrodes as possible, and at the same time not interfering with the electric characteristics of the capacitor electrodes and not interfering with the movement of the measurement diaphragm.
The sensor element is designed to comprise an integrated temperature measurement bridge, which makes a compensation of the drift of the sensor element possible for a change of the ambient temperature and for temperature changes of the measurement medium. The signal from the measurement bridge can be processed digitally, what increases the applicability of the temperature measurement bridge. Resistive bridge elements of thin film type are coated on an interior surface inside a sensor housing comprising a thick base plate and a thin plate, the interior surface being located between the base plan and the thin plate, so that the bridge is separated from the measurement electrodes only by the relatively thin plate.
A pressure sensor of substantially ceramic material thus comprises a rigid and stable, non-deformable house part consisting of a thicker base plate and an interior shielding plate, and furthermore it comprises a diaphragm having a portion movable with the pressure which is to be sensed or measured. A cavity comprising a reference pressure is formed between the shielding plate and the diaphragm. On the opposite walls of the cavity electrodes are arranged, which form a capacitor, the capacity of which can be sensed by electronic circuits. A temperature sensor comprises a bridge circuit. This circuit includes thermistors arranged inside the house part, between the base plate and the shielding plate, and reference resistors on that surface of the base plate which faces outwards. The shielding plate is thin, e.g., having a thickness substantially between the thickness of the diaphragm and twice that thickness. Thereby the thermistors are located near the diaphragm and are sensitive to the temperature thereof. This position of the thermistors also results in that the temperature sensor elements, particularly the thermistors, give a minimum influence on the electric properties of the pressure sensor and in particular on the mechanical properties of the diaphragm.
Generally, a pressure sensor can comprise a pressure sensor house assembly made of substantially ceramic materials. The assembly comprises a substantially rigid house part having no movable portions and an at least partly movable diaphragm. A cavity is formed between the house part and the diaphragm. At least one temperature sensor or temperature sensing element is arranged in the interior of the house part, i.e inside the material of the house part. The temperature sensing element is thus not in contact with the exterior of the assembly and not in contact with the cavity. The temperature sensing element is preferably located at or very near the cavity and can be separated therefrom only by a ceramic plate. This ceramic plate is advantageously a thin plate, having a thickness substantially equal to the thickness of the diaphragm or at most equal to twice that thickness. Preferably, the temperature sensor is also located at the periphery of the house part in order not to interfere with electrical fields at central portions of the house part and the diaphragm.
The temperature sensing element can be arranged between a thicker base plate and a thinner shielding plate, the latter of which has a surface, which forms a wall in the cavity, wand in particular it can be arranged between an interior surface of the base plate and a joint, which attaches the base plate to the shielding plate
An electrically conducting, shielding layer can be located between the base plate and the shielding plate and preferably centrally in the contact surface between the base plate and the shielding plate. Then the shielding layer and the temperature sensor may be located separately from each other, as seen in directions in the contact surface between the base plate and the shielding plate.
The area of an electrode, which is centrally located on the surface of the housing part at the cavity, may be somewhat smaller the e area of an opposite electrode, which is located centrally on the surface of the diaphragm at the cavity. An electrode, which is centrally located on the surface of the diaphragm at the cavity, is preferably surrounded by a substantially annular shielding layer made of an electrically conducting material.
The pressure sensor has typically the shape of a plate such as a substantially circular plate and then house part also has the shape of a plate with the same exterior form. Then two temperature sensor elements are advantageously arranged opposite each other along a diameter and symmetrically in the house part in relation to a central axis of the house part, the central axis being perpendicular to the large surface of the house part.
Furthermore, at least one reference resistor can be applied to or at an exterior surface of the house part. For a plate-shaped pressure sensor such as a substantially circular plate two reference resistances can be applied opposite each other along a diameter and symmetrically on or at the house part respectively in relation to a central axis of the house part.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.